1. Field of the Invention
The present invention relates to a silver halide photographic emulsion in which a specific wave length range is highly spectrally sensitized by a supersensitization, more especially, the present invention relates to a silver halide photographic emulsion in which the wave length range from 580 to 640 nm is spectrally sensitized.
2. Description of the Prior Art
An appropriate spectral sensitivity distribution over the visible wave length range to which human eyes are sensitive is given to conventional photographic sensitive materials. An appropriate spectral sensitivity distribution is required for suitable color reproduction. Usually, a combination of two or more sensitizing dyes is selected, the combination having a supersensitization effect.
To obtain excellent color reproduction, it is not preferred to have high sensitivity at too long a wave length, e.g. a wave length longer than 660 nm (the wave length at the maximum sensitization) or to have high sensitivity only at too short a wave length region, e.g., shorter than 580 nm (the wave length at the maximum sensitization). It is difficult to selectively increase sensitivity within the wave length region from 580 to 640 nm, and the solution to this problem is an important matter in this technical field.
U.S. Pat. No. 3,338,714 Ficken et al. discloses dye combinations. However, in Dye II of Ficken et al., the N-substituent R.sub.12 on the benzoxazole nucleus is defined as an alkyl group and the N-substituent R.sub.14 on the benzothiazole, benzoselenazole, naphthothiazole or naphthoselenazole nucleus is defined as a group A--Q wherein A is an alkylene chain, and Q is an amide, carboxylic acid or sulfonic acid. The N-substituent (R.sub.2) of the benzoxazole nucleus (Z.sub.2) of the present invention represents a hydroxyalkyl group, a carboxyalkyl group or an alkyl group having a sulfo group, and the N-substituent (R.sub.1) on the .beta.-naphthothiazole or .beta. -naphthoselenazole (Z.sub.1) nucleus represents an unsubstituted alkyl group or a substituted alkyl group, preferably an unsubstituted alkyl group or an alkoxyalkyl group.
The emulsions disclosed in U.S. Pat. No. 3,463,640 Ficken et al. comprise a dye of the formula (III) having a benzoxazole nucleus, together with either (a) two dyes of formula (I) or (b) a combination of a dye of the formula (I) and a dye of the formula (II) as essential components, i.e., the emulsion must contain a combination of three dyes. On the contrary, the emulsion of the invention comprises a combination of two dyes and does not necessarily comprise three dyes. Further, the carbocyanine dye having a benzoxazole nucleus used in the present invention contains, as an essential moiety, a .beta.-naphthothiazole (or .beta.-naphthoselenazole) nucleus, whereas dye (III) of the Ficken et al reference contains a benzothiazole (or benzoselenazole) nucleus and does not contain a .beta.-naphthothiazole or .beta.-naphthoselenazole) nucleus.
The present invention uses a carbocyanine dye having a benzoxazole nucleus as Dye I, whereas U.S. Pat. No. 3,679,428 Shiba et al uses a carbocyanine dye having a naphthoxazole nucleus. The dyes of Shiba et al show inferior color staining.